Dennerle made a great Substrate Calculator:

Just enter the dimensions of your tank, the substrate height you like, and if you need to include plant growth media or not (just tick the "agar box"). It will show you the rough amount of Dennerle substrates you would need. This is not limited to Dennerle products and can be easily applied as guideline to other gravels and substrates (e.g. Unipac).